Sign determination of dipolar couplings in liquid crystals by off-magic-angle sample spinning

Highlights: • Solid-State NMR. • J-resolved MAS spectra. • Sign of dipolar coupling. • Resolving ambiguities in structural and dynamic parameters. Dipolar NMR spectroscopy is a powerful analytical tool to study structural and dynamic properties of rigid and soft solids at the molecular level. While magnitudes of dipolar couplings can be determined from spectral splittings, signs of the constants are unavailable. A method to determine the signs of the dipolar couplings in liquid crystals is presented. Sign information is directly obtained from one-dimensional NMR spectra in samples spinning at an angle slightly misset from the magic angle. The approach is used to develop an experimental strategy for sign-sensitive measurements of heteronuclear dipolar couplings in anisotropic soft materials.